This invention concerns combatting air pollution from the exhaust gas of a lean burn engine. In particular, it concerns apparatus for, and a method of, reducing the content of nitrogen oxides (NOx) in such gas.
Lean burn engines (which have an air-fuel ratio greater than 14.7, generally in the range 19-50) exhibit higher fuel economy and lower hydrocarbon emissions than do stoichiometrically operated engines and are increasing in number. Emissions from diesel engines are now being regulated by legislation, and whilst it is not too difficult to meet regulations on hydrocarbon or CO emissions, it is difficult to meet regulations on NOx emissions. Since exhaust gas from lean burn engines such as diesel engines is high in oxygen content throughout the engine cycle, it is more difficult to reduce NOx to nitrogen than in the case of stoichiometrically operated engines. The difficulty is compounded by the lower gas temperature. Various approaches are being considered to reduce NOx under the oxidising conditions. One approach is that of selective catalytic reduction (SCR) with hydrocarbon, but a catalyst of sufficient activity and durability to achieve the required conversion has not been found. Another approach is to adsorb the NOx by an adsorbent when the exhaust gas is lean (ie when there is a stoichiometric excess of oxygen) and release and reduce the adsorbed NOx when the exhaust gas is rich, the exhaust gas being made rich periodically. During the lean operation, NO is oxidised to NO2 which can then react readily with the adsorbent surface to form nitrate. This approach, though, is constrained at low temperature by restricted ability to form NO2 and by adsorbent regeneration and at high temperature by sulphur poisoning. Most adsorbents operate in a certain temperature window and are deactivated by sulphate formation. The approach of the present invention is that of SCR of NOx by NH3. This approach has been applied to static diesel engines using a V2O5xe2x80x94TiO2 type catalyst.
The application of NH3 SCR technology to the control of NOx emission from lean burn vehicles, however, requires a suitable NH3 supply strategy, especially at low temperature, for various reasons. The engine-out NOx varies with temperature, so the amount of NH3 supplied must be well controlled as a function of the temperature to maintain the appropriate stoichiometry for the reaction; an insufficient supply of NH3 results in inadequate NOx reduction, whilst an excess may cause NH3 to slip past the catalyst Whilst at sufficiently high temperature, the catalyst can selectively oxidise that excess NH3 to N2, at low temperature, the unreacted NH3 will be emitted as such. Even if the proper stoichiometry of NH3 is provided, the catalyst may not be sufficiently active at low temperature to react all the NH3 with the NOx. For example, FIG. 1 shows the reaction of NH3 with NOx over a non-metallised zeolite as a function of temperature at a stoichiometry of 1:1 at an inlet concentration of 200 ppm. It can be seen that at temperatures below 300xc2x0 C. the reduction does not proceed to any significant extent. Furthermore, it has been reported that the presence of excess NH3 at low temperature could lead to the formation of NH4NO3 and (NH4)2SO4. There is also evidence that the presence of excess gas phase NH3 can inhibit the NH3 SCR reaction over some catalysts at low temperature. Urea is usually the preferred form of storing NH3 on a vehicle. Urea is readily available and is stable in water solution. However, it only hydrolyses readily to NH3 at temperatures greater than 150xc2x0 C., and may not be a suitable source of NH3 at low temperature. Exhaust gas temperatures, though, vary over an engine cycle and for the average light duty diesel car a significant fraction of that cycle is at low temperature. Thus, the control of NOx at low temperature is a problem.
Methods have been suggested to mitigate this problem. For instance, U.S. Pat. No. 5,785,937, JP-A-07136465 and U.S. Pat. No. 4,963,332 all suggest the use of ammonia as a reductant to convert NOx to nitrogen over a catalyst EP-A-0773354 also describes the use of ammonia to reduce NOx to nitrogen. However, ammonia is synthesis in situ over a three-way catalyst during the rich burning phase of the engine and the supply of ammonia is triggered as a function of the stoichiometry of the fuel in terms of the fuel to air ratio not as a function of temperature.
The present invention provides an improved apparatus and method for reducing the content of NOx.
Accordingly, the invention provides an apparatus for reducing the content of nitrogen oxides (NOx) in the exhaust gas of a lean burn engine, which apparatus comprises:
(a) an exhaust capable of allowing exhaust gases to flow therethrough;
(b) a selective catalytic reduction catalyst located in the flow-path of the exhaust gas and being capable of (i) catalysing the reduction of the NOx by ammonia to nitrogen and (ii) adsorbing and desorbing ammonia during the engine cycle;
(c) means for supplying ammonia from an ammonia source to the catalyst; and
(d) switching means for intermittently supplying ammonia during an engine cycle thereby enabling (i) the catalyst to adsorb ammonia when ammonia supply is switched on and (ii) the adsorbed ammonia to react with NOx when ammonia supply is switched off,
characterised in that the catalyst comprises a zeolite and the switching means is triggered on and off at pre-set temperature levels of the catalyst.
The invention provides also a method of reducing the content of nitrogen oxides (NOx) in the exhaust gas of a lean burn engine, which method comprises passing the exhaust gas over a selective catalytic reduction catalyst which catalyses the reduction of the NOx by ammonia to nitrogen and which adsorbs and desorbs ammonia during the engine cycle, ammonia being supplied intermittently to the catalyst during the engine cycle, the catalyst adsorbing ammonia during its supply and the ammonia which has been adsorbed reacting with the NOx when the ammonia is not supplied.
We have discovered that ammonia can be absorbed on a SCR catalyst and thereafter used in the NOx reduction when ammonia is not being supplied. It is an advantage to be able to achieve the NOx reduction while supplying the ammonia intermittently. In particular, the ammonia supply can be halted and yet NOx reduction occur when the temperature of the catalyst is low and supply would have the problems referred to above. The stored ammonia can be used as reductant for NOx over the same catalyst without the presence of gas phase NH3.
The ammonia can be supplied without the exhaust gas so that the catalyst adsorbs the ammonia and then the exhaust gas passed over the catalyst for the NOx reduction to occur Preferably, however, the exhaust gas is passed continuously over the catalyst.
The invention uses adsorption and desorption characteristics of the required catalyst. A higher amount of NH3 will be adsorbed, and hence be available for subsequent reaction, if adsorption is at a lower temperature at which the catalyst adsorbs less NH3. Preferably NH3 is adsorbed at a temperature at which a large amount is adsorbed; the temperature is preferably below that of maximum desorption. The temperature, however, is preferably above that at which any significant formation of ammonium salts occurs. FIG. 2 shows the desorption profile from zeolite ZSM5 (non-metallised) of NH3 which had been pre-adsorbed at 100xc2x0 C. It can be seen that at say 300xc2x0 C. more NH3 is retained, adsorbed, than at say 400xc2x0 C., and that the temperature of maximum desorption is about 370xc2x0 C. Bearing in mind that the desorption of NH3 is endothermic, it can also be seen that if NH3 were adsorbed at say 300xc2x0 C. and then heated, NH3 would be desorbed in accordance with the graph so that less would be available for subsequent reaction, while if NH3 were adsorbed at the same temperature, 300xc2x0 C., and cooled, NH3 would not be desorbed so the adsorbed NH3 would be available for subsequent reaction. NH3 stored on the ZSM5 catalyst at 250xc2x0 C. can effectively be used to reduce NOx at a temperature as low as 150xc2x0 C. under exhaust conditions simulating those of a light duty diesel car. FIG. 3 shows the NH3 uptake of ZSM5 catalyst (non-metallised) from a gas mixture containing 4.5% CO2, 12% O2, 4.5% H2O, 200 ppm CO, 100 ppm C3H6, 20 ppm SO2 and 200 ppm NH3 with the balance N2 at 250xc2x0 C., and FIG. 4 shows the subsequent reaction of that adsorbed NH3 with NOx at 150xc2x0 C. It can be seen that significant amounts of NOx are reduced by the adsorbed NH3 over a period of time and that as the stored NH3 is being consumed, the reduction reaction declines with time. When the temperature rises in the engine cycle, however, NH3 can be applied again, and hence adsorbed NH3 replenished. Accordingly, the problem of applying NH3 at low temperature can be overcome by halting its supply and using adsorbed NH3. The amount of NH3 adsorbed on a fixed weight of catalyst can be increased by including its partial pressure in the gas mixture. For example, Table 1 gives the amount of NH3 adsorbed by a zeolite at 250xc2x0 C. from a simulated gas mixture of differing NH3 concentrations.
The means to make the supply of ammonia intermittent during the engine cycle in the present apparatus can be a switch which switches the ammonia supply on and off dependent on the level of NOx conversion occurring over the SCR catalyst. Preferably, however, the means to make the supply of ammonia intermittent comprises a switch to switch on the means to supply the ammonia when the temperature of the catalyst rises above a set level (i) during the engine cycle, and to switch off the means to supply the ammonia when the temperature of the catalyst falls below a set level (ii). The set level (i) is preferably in the range 250-400xc2x0 C., especially in the range 250-350xc2x0 C. The set level (ii) is preferably in the range 200-250xc2x0 C.
The ammonia can be supplied for instance 1-30 times per minute.
The source of ammonia and means to supply it from the source to the catalyst can be conventional. Compounds of ammonia as a solid or a solution in water are preferred. The compounds are preferably urea or ammonium carbamate. The means to supply the ammonia from the source to the catalyst can be a pipe through which it is injected into the exhaust gas up-stream of the catalyst. Thus, the present invention can be employed to provide a method of promoting the conversion of NOx under oxidising conditions in an exhaust fitted with a means of injecting NH3 and a catalyst which adsorbs NH3 during parts of the engine cycle in which the exhaust gas is sufficiently warmed for the hydrolysis of NH3 precursor and injection of ammonia and ammonia is adsorbed by the catalyst for use as reductant for NOx during parts of the engine cycle in which the exhaust gas is cooler, without the need for the continuous injection of NH3 into the exhaust gas.
It can be seen that the invention provides an exhaust system for an engine operating generally under lean conditions, which exhibits a higher exhaust gas temperature and a lower exhaust gas temperature, the lower exhaust gas temperature being inadequate for the effective hydrolysis of NH3 precursor and injection of NH3 (generally a temperature below 200xc2x0 C.), and an NH3 SCR catalyst arranged and constructed so that during the higher exhaust gas temperature parts of the engine cycle the catalyst adsorbs NH3 and during the lower exhaust gas temperature parts of the engine cycle the adsorbed NH3 is used as reductant for NOx.
The catalyst can be any which has the required characteristics of the present catalyst. The same material can both selectively catalyse the reduction and also adsorb and desorb the ammonia, and this is preferred. However, different materials in the catalyst can perform the two functions, one material catalysing and one material adsorbing and desorbing. When different materials are employed, they can be physically separate or, preferably, in admixture one with another. A zeolite can perform both functions or a zeolite can be employed which performs one function together with a different material, which may or may not be a zeolite, which performs the other function. The catalyst preferably comprises a zeolite. The zeolite can be metallised or non-metallised, and can have various silica-to-alumina ratios. Examples are metallised or non-metallised ZSM5, mordenite, xcex3 zeolite and xcex2 zeolite. Preferred is ZSM5 or ion-exchanged or metal impregnated ZSM5 such as Cu/ZSM5. It may be desirable that the zeolite contains metal, especially Cu, Ce, Fe or Pt; this can improve the low temperature SCR activity. The zeolite can contain for instance 1-10% of metal by weight. The catalyst should have an appropriate structure, for instance in terms of pore size or surface acid sites, to trap and release NH3.
The catalyst is preferably carried out on a support substrate, in particular a honeycomb monolith of the flow-through type. The monolith can be metal or ceramic. The substrate can be conventional.
Nitrogen oxide (NO) is usually the most abundant nitrogen oxide in an engine exhaust stream, but at lower temperatures the reaction of the adsorbed NH3 on a zeolite catalyst occurs more readily with NO2 than with NO. Accordingly it is often desirably to oxidise NO to NO2 up-stream of the SCR catalyst, particularly at low temperature.
The present engine can b a diesel or petrol (gasoline) engine. The diesel engine can be a light duty or heavy duty diesel engine. The engine is preferably that of a vehicle.